The tensile performance of high-entropy nanocrystalline- and single crystal-CoNiCrFeMn alloy, as well as polycrystalline- and single crystal-Ni metal, was comparatively assessed, while the evolution of their microstructures and the deformation induced difects such as dislocations, voids and cracks etc. with the deformation process and temperature was searched in an attempt to reveal the relationship between their mechanical performance and the aforesaid evolution. Results show that when the temperature lifting from 10 K to 1000 K, the yield stress of the high-entropy nanocrystalline CoNiCrFeMn alloy decreases by 14.9%, 13.1% and 17.4%, whose corresponding temperature is 10 K, 300 K and 1000 K respectively, in comparision to those of the high-entropy single crystal ones; While the tensile strength of the polycrystalline Ni decreased by 38.9%, 30% and 32.3% of that for single crystalline Ni, whose corresponding temperature is 10 K, 300 K and 1000 K respectively; Likewise, the elastic modulus and yield strength of the high entropy nanocrystalline alloy and nanocrystalline nickel decrease linearly with the increasing temperature. However, the overall decrease percentage of the value for yield stress of the polycrystalline nickel is greater than that of the high entropy single crystal alloy, owing to the exist of internal stresses, cracks and cavities induced by grain boundary defects of the former. It is thought that the geometry shape and size of the formed cavities and cracks are the fundamental cause responsible to the sharp decline of the mechanical properties of the similar materials in practical application, and also to the significant difference of the tensile mechanical properties between the high entropy nanocrystalline alloy and the nanocrystalline nickel. The applied tensile load may result in the formation of a large number of stacking faults within grains of polycrystalline materials, and thus the large grains are easy to be differentiated into fine grains with the increasing temperature, in other word, to realize the grain refinement. In addition, the high entropy polycrystalline alloy and polycrystalline nickel are more likely to generate latest dislocations at grain boundary edge induced by internal stresses, hence, the dislocation distribution is consistent with the internal stress distribution. With the increasing temperature, the distribution area of grain boundaries within polycrystalline materials will be further expanded due to thermal expansion, therefore, the area with internal stresses will enlarge accordingly, in comparison to that at lower temperature.